Captain Sim, thanks for the headsup about turning off FSX, that's what was causing values to default.  I've continued experimenting with the csweapon_mg_shot1.wav line in CSWeapon.xml  I changed the loopEnd= value to "0.0"  When I fired the M60 it then made a single shot sound, which is what the wave file is, and the rounds counter counted down as long as I held the button but no further soound.  There was no muzzle flash, however.  I then added loopBeg="0.0", arbitrarily trying different values since I was shooting in the dark (sorry for the bad pun), and then I had the single shot sound, a single muzzle flash and the rounds counted down, but it still wouldn't repeat the sound and muzzle flash to a rata-tat-tat-tat.  It's possible that with the correct loopBeg value it may work correctly.
Also, I wondered if FSX SP2 versus Acceleration is the variable that determines who has gun sound problems and who does not.  I am running SP2 and I have the sound problem.  How about you others who have reported it, and what about Mark who did not?